Optical nano-spirals could help fight counterfeits

Researchers have created the world’s smallest spirals with unique optical properties that could be embedded in to valuable objects to prevent counterfeit.

The spirals, made of gold and six million times smaller than a dime, have been put through a series of tests using ultrafast lasers at Vanderbilt University, USA, and the Pacific Northwest National Laboratory in Richland, USA.

"They are certainly smaller than any of the spirals we've found reported in the scientific literature," said Roderick Davidson, the Vanderbilt doctoral student who perfected a way to study the optical behaviour of the spirals.

The minuscule size is the key to the spirals’ unique properties. Smaller than the wavelength of visible light, the spirals, for example, emit visible blue light when illuminated with infrared laser light. This effect was previously described in some crystals. The nano-spirals, in fact, produce blue light four times stronger than the best-performing crystals.

"This is similar to what happens with a violin string when it is bowed vigorously," said Richard Haglund, Stevenson Professor of Physics, who directed the research. "If you bow a violin string very lightly it produces a single tone. But, if you bow it vigorously, it also begins producing higher harmonics, or overtones. The electrons at the centre of the spirals are driven pretty vigorously by the laser's electric field. The blue light is exactly an octave higher than the infrared - the second harmonic."

The nano-spirals also have a distinctive response to polarised laser light. When struck by a beam of linearly polarised light, the amount of blue light the nano-spirals emit varies as the angle of the plane of polarization is rotated through 360 degrees. The effect is even more dramatic when circularly polarised laser light is used.

These features, the researchers believe, would make the spirals an ideal tool for producing inimitable marks for valuable objects including credit cards, notes or identity documents.

"If nano-spirals were embedded in a credit card or identification card, they could be detected by a device comparable to a barcode reader," said Haglund.

The spirals would be invisible to the human eye but emit strong enough effect when illuminated to allow for secure identification. The researchers proposed the spirals could be placed in a secret location on the card, providing an extra security layer.

The researchers also argue that coded nano-spiral arrays could be encapsulated and placed in explosives, chemicals and drugs - any substance that someone wants to track closely - and then detected using an optical readout device.

The spirals were designed and made at Vanderbilt by another doctoral student, Jed Ziegler, now at the Naval Research Laboratory. The results of the study were reported in the latest issue of the Journal of Nanophotonics.